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A curiosity, but why do the mounting holes on PCBs have those small holes on the outer chainring?

I mean these: enter image description here

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  • \$\begingroup\$ Those are vias to connect the outer layer seen in the image to other layers. \$\endgroup\$
    – winny
    Mar 14, 2023 at 9:00

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They are for electrical conductivity.

They allow to connect the copper area of the mounting hole electrically between different copper layers, to have a good connection from the metallic screw on top and metallic chassis mounting stand to ground plane.

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    \$\begingroup\$ If good connection from the metallic screw on top and metallic chassis is required then why isn't the fastener hole plated thru ? \$\endgroup\$
    – D Duck
    Mar 14, 2023 at 9:19
  • \$\begingroup\$ @DDuck maybe because that could eventually wear off. Which is an extremely unlikely scenario... \$\endgroup\$
    – tobalt
    Mar 14, 2023 at 9:22
  • \$\begingroup\$ @DDuck That is a very valid question. \$\endgroup\$
    – winny
    Mar 14, 2023 at 9:23
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    \$\begingroup\$ The question electronics.stackexchange.com/questions/137394/… shows a photo of a variety of mounting holes, one with a ring of vias surrounding a plated thru hole and one without. And pretty well explains that the main hole isn't plated thru to avoid small metal particles being generated -- and that the vias are for electrical conductivity and mechanical stiffness to avoid the bearing pad from lifting off. \$\endgroup\$
    – D Duck
    Mar 14, 2023 at 9:27
  • \$\begingroup\$ @DDuck Impossible to know why. There might be a good reason but we don't know why that board does not have a large plated through mounting hole. The nearest PCB that I am looking at does have a large plated through mounting hole. \$\endgroup\$
    – Justme
    Mar 14, 2023 at 9:29

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